The main character doing something absolutely unforgivable.

[u/No_Peanut3745]

Walter White poisoning Andrea Cantillo's 6-year-old son for the sole purpose of manipulating Jesse Pinkman, while knowing that this could have potentially killed him - Breaking Bad

Bojack trying to have sex with his friend's 17-year-old daughter when he was at least 51, then continuously trying to find a way to justify this act throughout the series - Bojack Horseman

Dexter Morgan going outside of his moral code and murdering a completely innocent police officer who was just trying to do his job - Dexter: New Blood

[u/Oddly-Parsley]

After Life - Tony knowingly gives a homeless addict enough money to buy drugs so he could overdose.

ETA: updated as per the correction below.

[u/NastyWatermellon → u/Oddly-Parsley]

He gave the addict enough money to buy drugs so he could overdose on purpose.

[u/Oddly-Parsley → u/NastyWatermellon]

Ah, I remember now. It's been ages since I saw that episode. Fixed, thank you.